Method and apparatus for generating a continuous beam of neutral atoms

ABSTRACT

Method and apparatus for generating a continuous beam of sputtered neutral atoms that are sputtered from metallic members, condensed layers of gases, single crystals and condensed layers of gases on single crystals, and which continuous beam passes through an aperture providing a directed beam that may be restricted to neutral atoms having a monovelocity by a velocity selector means.

United States Patent [72] Inventors Beniamin B. Meckel La Mesa; Bernd H. Richelmann, San Diego, Calif. [21] Appl. No. 500,580 [22] Filed Oct. 22, 1965 [45] Patented Mar. 9, 1971 [73] Assignee Physics Technology Laboratories Inc.

La Mesa, Calif.

[5 4] METHOD AND APPARATUS FOR GENERATING A CONTINUOUS BEAM OF NEUTRAL ATOMS 5 Claims, 3 Drawing Figs.

[52] US. Cl. 250/84, 204/192, 204/298, 250/41.3, 250/42 [51] Int. Cl H01j 37/00 [50] Field ofSearch 250/41 .3, 49.5 (0), 495(1), 495 (8), 41.9 (1813), 42, 84; 204/192, 298

[56] References Cited UNITED STATES PATENTS 2/1965 Kagan 3,180,987 4/1965' Cunningham et a1 250/4 9 .5 3,333,982 8/1967 Horn et al 204/298X FOREIGN PATENTS 829,783 3/ 1960 Great Britain 204/298 OTHER REFERENCES Improved Slotted Disk Type Velocity Selector for Molecular Beams by H. U. Hostettler et al. from The Review of Scientific Instruments vol. 31, No. 8, August, 1960, pages 872- 877.

Primary Examiner-William E. Lindquist Attorney-Carl R. Brown ABSTRACT: Method and apparatus for generating a continuous beam of sputtered neutral atoms that are sputtered from metallic members, condensed layers of gases, single crystals and condensed layers of gases on single crystals, and which continuous beam passes through an aperture providing a directed beam that may be restricted to neutral atoms having a monovelocity by a velocity selector means.

PATENEEDHAR em 3.569.706

'SHEET 1 OF 2 F IE.

METHOD AND APPARATUS FOR GENERATING A CONTINUOUS BEAM OF NEUTRAL ATOMS BACKGROUND OF THE INVENTION Recent scientific efforts and discoveries have made the investigation of particle-surface interactions interesting from an academic as well as a practical application point of view. The processes involved in the energetic impact of particles such as ions, neutral atoms and the like are of particular interest in simulating and then analyzing certain space physics phenomena. Immediate knowledge is of a direct interest for medium energy impacts that are characteristic of what may be expected to be encountered by satellites and missiles in the upper atmosphere or in space. Such phenomena in particle surface interactions as exitation, ionization, recombination processes as well as accommodation coefiicients, sticking probabilities, reflection characteristics, and numerous other values and phenomena are of particular interest. Therefore a need exists for a efficient, reliable and inexpensive means for producing a continuous beam of neutral atomswhich beam comprises particles sputtered from diverse substances to produce energies in given ranges. It is of further advantage to produce a dense continuous beams having a monochromatic velocity.

Therefore it is an object of this invention to provide a new and improved method and apparatus for generating particle beams.

It is another object of this invention to provide a new and improved method and apparatus for producing a continuous beam of neutral atoms in a vacuum.

It is another object of this invention to provide a new and improved method and apparatus for generating a continuous beam of neutral atoms having monochromatic velocities.

It is another object of this invention to provide a new and improved method and apparatus for generating a continuous beam of neutral atoms having desired monoenergetic levels and desired selective monochromatic velocities.

lt is another object of this invention to provide a new and improved method and apparatus for generating a neutral atomic beam by sputtering with substantially reduced interference from the plasma used in the sputtering.

It is another object of this invention to provide a new and improved method and apparatus for producing a neutral atom beam in a vacuum having monoenergetic levels or monochromatic velocities, from condensed layers of gases.

It is another object of this invention to provide a new and improved method and apparatus for producing particle beams from gases that are noncondensible at room temperature.

The method and apparatus of our invention involves producing continuous neutral atomic beams by sputtering; that is by bombarding the surface of a solid material with energetic ions. This causes atoms in the solid to be ejected with average energies that are two orders of magnitude above thermal. These ejected particles will leave in directions corresponding to the alignment of atoms in the solid that is sputtered. Where the lattice of the crystal has been crystallographically oriented so that the close-packed atom rows are aligned parallel to the desired beam direction, then the ejected neutral atoms will move substantially in a given direction. In the aforesaid process a solid is placed in a vacuum containing plasma. The charged ions in the plasma impact the solid and by sputtering cause neutral atoms to be ejected therefrom. An aperture means is placed immediately adjacent the surface of the solid and passes a continuous beam therethrough. A velocity selector means then receives the beam through the aperture and passes only those neutral atoms that have a given selected velocity. Thus a continuous beam of neutral atoms having a given monochromatic velocity is produced.

While any solid metallic material may be sputtered, it is often desirable to have particular metallic particles in the beam. Thus selective metals must be used. If there is no particular alignment of the atoms in the metal, then the neutral atoms are ejected from the surface in a random pattern. However, where the metal is a single crystal, then the neutral atoms are ejected in a particular direction. The directed beams, when aligned with the aperture means, passes therethrough a beam considerably more dense than that obtained from a random pattern.

In other applications it is particularly useful to produce particle beams from gases that are noncondensible in normal atmosphere. To accomplish this a Cold Finger" is used to lay a condensed layer of gas, such as for example nitrogen or oxygen, on a cold solid surface, such as for example copper, in a vacuum. A neutral atomic beam is then created by sputtering particles of the condensed gas. It has been found that the layers of atoms of the gas on the solid assume a somewhat similar orientation with the lattice or alignment of the atoms in the solid. Where there is random alignment of the atoms, then the neutral atomic beam generatedfrom sputtering the condensed gas layer will have no particular directional orientation. However, where the solid used as the Cold Finger" is a single crystal with atom rows aligned parallel to the desired beam direction; then it has been found that the layers of gas condensed on the single crystal surface follow the underlying atomic alignment of the single crystal and gas particles ejected by sputtering leave in a preferred direction. Thus a dense neutral atomic beam of gas particles may be directed through the aperture means.

It should be recognized that any material capable of being vaporized may be sputtered and supply particles in a continuous neutral atomic beam. Further a dense neutral beam may be created from the vaporized material on a single crystal sur face.

These and other objects and features of this invention will become more apparent from the following description and accompanying drawings, wherein:

FIG. 1 is a side view of schematic representation partly in section of an embodiment of this invention.

FIG. 2 is an enlarged cross-sectional view of the neutral atomic beam generating apparatus.

FIG. 3 is a schematic representation of the sputtering of gaseous molecules laid onto a single crystal copper surface.

Referring now to FIG. 1 there is shown on the right a container 18 that houses the means for producing a neutral atom beam by sputtering. The container 18 may be made of glass or other suitable material. The container 18 has a vacuum drawn therein by a diffusion vacuum pump driven by motor 62 that evacuates volume 24 and through aperture means 20 evacuates volume 22. Also mounted in housing 18 is a cold trap" that comprises a container 36 for holding a very cold liquid such as liquid nitrogen, oxygen or the like. Valve 38 closes and opens container 36 for filling the container and to provide a means for removing boil off. A compartment 42 is mounted in the side of container 36 for holding the metallic solid 40 to be sputtered. The compartment prevents adverse chemical reactions between the cold liquid and the particular metallic solid 40. Further the metallic solid 40 may be selectively removed or changed without required removal of liquid 37.

The very cold liquid or liquified gas 37 has a temperature sufficiently low to reduce the temperature of the solid member 40 below the condensation temperature of the gas to be sputtered.

As previously stated small quantities of a gas composition that may be oxygen or nitrogen is supplied through valve 30 and pipe 28 to the evacuated volume 22 of glass container 18. This gas will then be condensed on the end 76 of the cold finger" 40, forming layers 78 of the condensed gas thereon.

Coils 34 carry RF energy for creating plasma within volume 22 from the gas received from conduit 2%. A potential source 50 provides a negative potential through conductor 51 to the metallic solid 40 making end 76 a cathode. Positive potential is supplied to metal flange 52 through line 49. Thus the plasma in volume 22 created by RF coils 34 has a positive potential and positive ions impact against the negative end 76 or the liquifred gas layers 78 to accomplish sputtering. Flange 52 connects the aluminum chamber 12 to the evacuated container 18 with a connection that is sealed with O-ring gaskets or the like (not shown). A tube 20 inserted through an aperture in flange 52, has an end 32 that is immediately adjacent the end 76 and 78 of the cold finger" 40. As an example only of dimensions; the aperture in tube 20 may be rectangular in cross section of l centimeter by 2 centimeters and the end 32 may be as close as 2 or 3 centimeters from end 76.

Container 12, which may be made of any suitable material such as aluminum or the like, houses a well known velocity selector in a vacuum drawn in the direction of arrow 64 by a known diffusion pump that is in turn backed by known ruffing pump 62. The velocity selector comprises a plurality of blades 48 mounted in spaced relationship along the length of rotor.

axle 56 that is turned by motor 58. Each of the blades 48 has a plurality of notches around the circumference, that when rotated only pass a particle beam having a given speed. A cross-electron beam detector 60 receives that portion of the particle beam that passes through the velocity selector and measures the number-density from which the flux density of the beam is calculated. The beam 44 and 46 may be passed on through aperture 61 into the evacuated chamber 26. An ionization gauge is used to determine the total background pressure and pressure increase from the beam. A gauge 68 indicates the vacuum in the system and ruffing pump 74 backs a second diffusion pump.

This invention has several modes of operation; all of which are easily understood from a description of the embodiment. The diffusion pump backed by ruffing pump 62 draws a vacuum in volume 24. This in turn evacuates volume 22 through tube 20. A gas such as oxygen or nitrogen is ejected in very small amounts through line 28 into volume 22. This gas diffuses through volume 22 and is in a position to be condensed in a thin layer on cold finger 76. Where the injected gas is oxygen, then the cold trap 36 may be filled with liquid nitrogen. If liquid nitrogen is the injected gas, then the cold trap may be filled with liquid helium. This assures that the cold finger 76 has a lower temperature than the condensation temperature of the gas in volume 22. RF energy is applied to coils 34, ionizing the gas in volume 22 and creating ions having a potential considerably more positive than the cathode or cold finger 76. Thus the positive ions bombard the gas molecules 78 on the end of the cold finger" 76 causing neutral atoms 79 to be ejected therefrom forming beams 79 and 44 of neutral atoms.

Assuming that the cold finger" 40 is made of normal copper, then'it will not be a single crystal and the neutral atoms ejected will adopt a random pattern 79 in movement therefrom. However, a portion of the atoms 44 will pass into tube 20 and move therethrough into volume 24 to the velocity selector. The speed of rotation of rotor axle 56 is established by adjusting the speed of motor 58 so that the notches in blades 48 will only permit a neutral atomic beam to pass therethrough having a given monochromatic velocity. The beam then impacts the cross-electron beam detector 60. Where desired as an alternative operation, the cross-electron beam detector 60 may be removed and the beam passed therethrough into volume 26 for impacting on or against articles therein, thus simulating particle contact space satellites and the like in space.

As previously stated the cold finger 40 may be a single wherein the atomic structure of the condensed nitrogen or oxygen layers on the surface follow the underlying atomic structure of the single crystal. Thus when the gas particles are ejected by sputtering, they leave in a preferential direction. This is illustrated in FIG. 3. The tube 20 not only passes neutral atoms of the beam that move in the given direction through the tube, but it also functions to suppress the plasma in volume 22 from the path along which the fast neutrals traverse towards the velocity detector. This reduces the plasma-neutral interactions that would otherwise produce excited neutrals, since plasma is efficient in detaching electrons from the fast moving neutrals. Where desired the temperature in the cold trap" may be raised to the pint that the metallic member 40 no longer functions as a cold finger." Then sputtering from the solid 40 is accomplished in the normal manner to obtain a nonoriented beam or where solid 40. is a single crystal to obtain an orientated beam 44.

This method and apparatus is invaluable to gas kinetic research, particle-surface investigations, and other studies where atomic beams with these parameters are required. The source can be modified to produce multimass neutral beams with particles having identical velocities but various energies. Accordingly this method and apparatus will mark a considerable advancement in accomplishing numerous laboratory and simulation problems as well as in the investigation of many basic reactions.

While there has been disclosed hereinabove certain preferred embodiment of this invention, it will be understood that various changes and modifications may be made therein without departing from the scope of the invention as defined in the appended claims.

We claim: 7

1. Apparatus for generating a dense continuous beam of neutral atoms from gases in preferred directions of movement and at a given speed comprising:

an evacuated housing enclosing a single crystal metallic member;

means for injecting a small amount of gas into said housing;

means for condensing portions of said gas on said metallic member;

means for ionizing a portion of said gas and sputtering said condensed gas by ion bombardment causing neutral atoms to be ejected in the preferred direction of said single crystal;

tube means aligned in said preferred direction for receiving and passing therethrough only the neutral atoms that enter said tube means in alignment therewith to form a neutral atom beam; and

velocity selector means for receiving said beam from said tube means and passing only that portion of said neutral atoms in said beam having a given speed.

2. Apparatus for generating a' continuous beam of neutral atoms from gases in preferred directions of movement and at a given speed comprising:

an evacuated housing enclosing a metallic member;

means for injecting a small amount of gas into said housing;

means for condensing portions of said gas on said metallic member;

means for ionizing a portion of said gas and sputtering said condensed gas by ion bombardment causing neutral atoms to be ejected therefrom;

tube means aligned for receiving and passing therethrough portions of said neutral atoms to form a neutral atom beam;

one end of said tube means being positioned immediately adjacent said condensed gas; and

velocity selector means for receiving said beam from said tube means and passing only that portion of said neutral atoms in said beam having a given speed.

3. The method of generating a continuous beam of neutral atoms from gases comprising the steps of:

drawing a vacuum in a container;

injecting a gas into said container;

condensing a portion of said gas on a cold finger in said container; ionizing the remainder of said gas in said container creating a plasma; and

generating a continuous beam of neutral atoms by using the plasma to sputter the condensed layers of gases on the cold finger surface in the vacuum.

4. The method of generating a continuous beam of neutral atoms from gases as claimed in claim 3 comprising the additional step of,

moving said continuous beam of neutral atoms through a restricted opening to provide a directed beam of neutral atoms.

neutral atoms that have a given selected velocity, thereby producing a continuous beam of neutral atoms having a given monochromatic velocity. 

2. Apparatus for generating a continuous beam of neutral atoms from gases in preferred directions of movement and at a given speed comprising: an evacuated housing enclosing a metallic member; meanS for injecting a small amount of gas into said housing; means for condensing portions of said gas on said metallic member; means for ionizing a portion of said gas and sputtering said condensed gas by ion bombardment causing neutral atoms to be ejected therefrom; tube means aligned for receiving and passing therethrough portions of said neutral atoms to form a neutral atom beam; one end of said tube means being positioned immediately adjacent said condensed gas; and velocity selector means for receiving said beam from said tube means and passing only that portion of said neutral atoms in said beam having a given speed.
 3. The method of generating a continuous beam of neutral atoms from gases comprising the steps of: drawing a vacuum in a container; injecting a gas into said container; condensing a portion of said gas on a cold finger in said container; ionizing the remainder of said gas in said container creating a plasma; and generating a continuous beam of neutral atoms by using the plasma to sputter the condensed layers of gases on the cold finger surface in the vacuum.
 4. The method of generating a continuous beam of neutral atoms from gases as claimed in claim 3 comprising the additional step of, moving said continuous beam of neutral atoms through a restricted opening to provide a directed beam of neutral atoms.
 5. The method of generating a continuous beam of neutral atoms from gases as claimed in claim 4 including the step of, employing a velocity selector device for receiving the beam through the restricted opening and passing only those neutral atoms that have a given selected velocity, thereby producing a continuous beam of neutral atoms having a given monochromatic velocity. 